Dock levelers or so-called dockboards are widely used in conjunction with freight vehicle loading docks and the like to provide a continuous path between the loading dock and the loadbed of a vehicle. One widely used type of dock leveler comprises a generally planar ramp or deck member which is pivotally connected to a frame disposed in a pit or recess in a loading dock and moveable between a stored position substantially coplanar with the surface of the loading dock and an elevated position preparatory to engagement with the loadbed of a vehicle when it is situated in proximity to the loading dock. So-called mechanical dock levelers typically utilize one or more heavy duty springs which operate through a linkage to bias the dock leveler ramp from the stored position to an elevated position, including the engaged or working position of the dock leveler with respect to the vehicle loadbed. Many mechanical and other conventional dock levelers are also operable to descend to a position below the surface of the loading dock to engage vehicles which have a low loadbed height.
In mechanical dock levelers of the general type mentioned above a hold-down mechanism must be utilized to maintain the dock leveler in the stored position or a desired working position against pivotal movement, except within a very limited range, about its hinge axis during operation or storage. Conventional hold-down mechanisms may be manually released to allow changing the position of the ramp about its hinge axis. However, a problem associated with conventional dock leveler hold-down mechanisms has become more acute as vehicle suspension mechanisms have been developed, including pneumatic suspension systems, which allow substantial variation in height of the loadbed, depending on the weight of the load supported thereon. Pneumatic suspension systems, in particular, are subject to movement within a fairly wide range of loadbed positions during loading and unloading. Accordingly, a dock leveler ramp engaged with a loadbed which moves considerably during cargo loading and unloading operations will itself tend to move with the loadbed as a result of the dock leveler lip or deck extension part being directly engaged with the vehicle loadbed.
If the dock leveler ramp begins to elevate substantially, any compensating linkage associated with the hold-down mechanism may "bottom out" or reach a limit position wherein further movement of the ramp will result in considerable damage or failure of the hold-down mechanism. Consequently, there has been a substantial need to develop a hold-down mechanism which is self releasing beyond a certain load condition imposed thereon to prevent damage to the dock leveler or the hold-down mechanism, or both, when the hold-down mechanism compensating linkage has moved to a limit position. It is to these ends that the present invention has been developed.